1. Field of the Invention
The invention relates to a controller for fill level sensors having a measuring electrode and a compensating electrode which both cooperate with a counter electrode for monitoring conditions in which the fill level of an electrically conductive liquid drops below a limit value.
2. The Prior Art
If the fill level of an electrically conducting liquid in a container, such as a steam boiler, drops below a lower limit value, a dangerous operating condition may arise, such as dry firing. The fill level of the liquid therefore has to be monitored in such installations so as to ensure it will not drop below the limit level.
Fill level sensors with a measuring electrode and a compensating electrode are preferably employed for such safety-relevant monitoring processes of the fill level. Both electrodes cooperate with a counter electrode. The measuring electrode serves to actually monitor the fill level. The compensating electrode is present in order to prevent error signals which may otherwise be caused by electrically conducting deposits or coatings that might form on the fill level sensors. As the counter electrode, it is possible to use, for example an electrically conductive tube that surrounds the other two electrodes with a spacing from the two electrodes (U.S. Pat. No. 3,910,118). However, it is also a common practice to let the electrically conductive container function as the counter electrode (DE-PS 25 31 915). The fill level sensors are connected to measuring instruments to produce a fill level signal in the event the fill level drops below the limit value.
In some applications, it is necessary to design the measuring instruments as self-monitoring devicesxe2x80x94see, for example the xe2x80x9cTechnische Regeln fxc3xcr Dampfkessel (TRD 604) [Technical Regulations for Steam Boilers]. The measuring instruments are required to automatically test, at preset time intervals, their capability of reliably producing the corresponding alarm signal if the fill level drops below the limit value.
The invention is concerned with the problem of providing a measuring instrument of the type specified above that has this self-monitoring capability.
The problem is solved according to the invention by a measuring instrument for fill level sensors having a measuring electrode and a compensating electrode that both cooperate with a counter electrode for monitoring conditions in which the fill level of an electrically conductive liquid drops below a limit value. The device has a measuring circuit and a control circuit connected to the measuring circuit. The measuring circuit connects the measuring electrode, the compensating electrode and the counter electrode. The control circuit produces an alarm signal if the fill level drops below the limit value. The measuring circuit has two series-connected test resistors forming a first branch of a bridge circuit. The measuring electrode, the compensating electrode and the counter electrode are connected to the measuring circuit so that the electrical measuring resistance developed between the measuring electrode and the counter electrode according to the fill level is connected in series with the electrical compensation resistance developed between the compensating electrode and the counter electrode. The two resistors jointly form the second branch of the bridge circuit.
In accordance with the invention, a diagonal line with a test bridge is provided between the two bridge branches. Control means for the test switch is also provided. The control means closes the test switch outside of preset test periods and opens the switch at preset test periods. The end of the diagonal line located between the two test resistors is connected to the base potential of the measuring circuit. The resistors of the bridge circuit are designed so that when the test switch is closed, the second bridge branch produces a signal determining imbalance of the bridge circuit. When the test switch is open, the imbalance of the bridge is determined by the first bridge branch, and a situation in which the fill level drops below the limit value is simulated, whereby the control circuit transmits an error signal if the corresponding signal of the measuring circuit is missing.
The test switch is closed and thus switched to passage while the fill level in the container is being monitored. The measuring resistance developed in the container, as well as the compensation resistance determine the unbalancing and consequently the output signal of the measuring instrument. When the fill level drops below the limit value, the measuring instrument issues a corresponding fill level signal. The measuring instrument also sends a signal if the fill level is above the limit value. The measuring instrument tests at preset time intervals its functionality as to whether an appropriate fill signal could be generated in case the fill level drops below the limit value. The test switch is opened for this purpose. The measurement resistance and the resistance to compensation are then of no importance to the unbalancing of the bridge, which is determined instead by the two test resistors that simulate the condition of xe2x80x9cfill level below limit valuexe2x80x9d. It must be possible in the test to produce in the control circuit the fill level signal xe2x80x9cshort of liquidxe2x80x9d. If this is not the case, an error is present and the measuring instrument emits a corresponding error signal. The test is not dependent on the conductivity of the liquid in the container.
In another embodiment, the ratio of the resistance values of the test resistors in the first bridge branch is smaller than the ratio of the resistance values of the measuring resistance and the compensating resistance, if both the measuring electrode and the compensating electrode are immersed in the liquid, and the sequence of the higher and lower resistance values in the two bridge branches is reversed. These features have a highly advantageous effect on the generation of a pronounced fill level signal.
In another embodiment, one test resistor has a higher resistance value than the measuring resistor when the measuring electrode is immersed. The other test resistor has a higher resistance than the compensating resistor when the compensating electrode is immersed. The resistance values of the test resistors are sufficiently high so that when the measuring electrode is immersed in the liquid, the second bridge branch dominates the unbalancing of the measuring bridge. These features provide a particularly strong unbalancing of the bridge circuit if the measuring electrode is immersed in the liquid. This strong imbalance is opposite to the unbalancing of the bridge caused by the test resistors. The formation of the signal is independent of any variations or differences that might occur in the supply voltage of the measuring instrument.
In another embodiment, the two bridge branches are connected to each other at their adjacent ends. Means are provided for measuring a first partial bridge voltage between the base potential and a connection on one end of the two bridge branches. Means are also provided for measuring a second partial bridge voltage between the base potential and the connection on the other end of the two bridge branches. In addition, a summing member is provided for adding up the two partial bridge voltages. The control circuit for producing the signal divides the sum of the two partial bridge voltages by the partial bridge voltage measured at the end close to the measuring resistor. The further development of the measuring instrument according to this embodiment offers a particularly advantageous determination of the unbalancing of the bridge and the production of the signal. The received signal is independent of any variations that might occur in the supply voltage of the measuring circuit. The production of the signal with digital means is facilitated as well.
The control of the tests as well as of the signal production can be realized especially advantageously according an embodiment in which the control circuit has an A/D-converter for the incoming signal voltages of the measuring circuit, and a microprocessor for controlling the test switch and for producing the signal.